1. Field of the Invention
This invention relates to a heating power source device for an apparatus for heating silicon to grow, and more particularly to a heating power source device adapted to supply a high voltage upon beginning of energization (at an initial part of heating) while it supplies a large electric current toward the end of energization (at a latter part of heating).
2. Description of the Prior Art
Referring to FIG. 1 which is a circuit diagram illustrating a typical one of conventional heating power source devices, a tapped transformer 1 has a primary winding 1a and a secondary winding 1b, and a pair of thyristor units 2 and 3 each including a pair of thyristors connected in anti-parallel relationship are connected to a high voltage tap and a low voltage tap of the transformer 1, respectively. A load 4 having a negative resistance characteristic such as of silicon is connected to be supplied electric power from the transformer 1 via the thyristor units 2 and 3.
Operations of the heating power source device will now be described. The load 4 presents a high resistance before it is heated, and hence the heating power source device must necessarily provide a high voltage initially. Accordingly, the thyristor unit 2 connected to the high voltage tap of the tapped transformer 1 is turned on to supply a high voltage to the load 4.
The heating power source device is controlled by an output signal from a controlling circuit not shown such that as the load 4 is heated to grow and its resistance becomes lower accordingly, the heating power source device energizes the load with an electric current corresponding to the growth of the load 4. However, as the supply voltage becomes lower, the power-factor becomes lower accordingly. Therefore, when the supply voltage to the load is lowered to a predetermined level, an output signal from the controlling circuit is supplied to the thyristor unit 3 connected to the low voltage tap of the tapped transformer 1 to switch the circuit to turn the thyristor 2 off to energize the load with a good power-factor until the load 4 is grown finally to such a degree as to allow a large electric current to flow therethrough.
Since the typical heating power source device is constructed as described above, while the high voltage tap is required to supply only a low electric current, the low voltage tap is required to supply a high electric current. Accordingly, the heating power source device has a defect that it requires a special transformer as well as high capacity thyristor units for controlling electric currents.